A fire at a three-story rowhouse on Rhode Island Avenue NW near North Capitol Street spread to two adjacent homes Monday morning and left 18 people homeless.

A neighbor reported the blaze to D.C. Fire and Emergency Medical Services about 5 a.m., saying there was a fire on the roof of a nearby home, officials said. No one was injured.

Pete Piringer, D.C. Fire and EMS spokesman, said that investigators were working to pinpoint the cause of the blaze but that it probably was accidental. He said the residents had earlier hosted a Fourth of July gathering on their rooftop deck.

The fire started on the unit block of Rhode Island Avenue NW and displaced six people from the home. Twelve residents living in adjacent houses were also left homeless, Piringer said.

One hundred firefighters responded to the blaze, and it was quickly brought under control, Piringer said. The damage was significant, he said, and might be valued at "a couple hundred thousand dollars."

The fire was the District's most significant of the holiday weekend, Piringer said. The agency received more than 700 calls Sunday -- typically the quietest day of the week, averaging about 450.

Piringer said between 8 p.m. Sunday and 8 a.m. Monday, the agency received 500 calls and occasionally confiscated fire crackers.

Most of the calls were grass, brush and dumpster fires related to fireworks, he said, and were considered larger risks because of the hot and dry weather conditions.